One is that it was cold and dry, contending that valley networks
and other geological features suggestive of liquid water in Mars' past were
essentially results of bursts of heat confined in space and time, suggesting
that Mars could not have sustained oceans. The other is that Mars was once warm
and wet, implying that it could once have supported lakes, seas and rainfall
for long periods.

Now
researchers have found evidence of icebergs on Mars, supporting a third idea of
the Red Planet's ancient climate ? that of a cold
and wet Mars, governed by oceans or seas covered partly in ice, as well as
glaciers and massive polar caps. [Photo evidence of past Mars icebergs.]

Boulder
and craters

To
peer into Mars' climatic past, scientists focused on the flat, smooth,
featureless Martian lowlands, which some have equated to an ancient ocean
basin.

However,
images captured by the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter
revealed the presence of boulders about 1.5-6.5 feet (0.5-2 meters) across, as
well as chains of roughly one or two dozen craters measuring 330-1,300 feet (100-400
meters) wide scattered throughout the northern plains. Both these details are
hard to reconcile with the notion of fine-grained sediments deposited on a deep
ocean basin, and had been used to cast doubts on the concept of an ocean
on Mars.

Now
astrobiologist Alberto Fairen at the SETI Institute and NASA Ames Research
Center and his colleagues suggest the presence and distribution of these
boulders and chains of craters could have been caused by rock fragments carried
by icebergs, a common process on Earth.

They
suggest glaciers in the highlands could have eroded the terrain, transporting
rock within them and on their surfaces. Armadas of icebergs would have formed
at the edges of glaciers as they melted and broke apart, which could then float
thousands of miles on the ocean before they disappeared, depositing rock
downward.

Also,
on Earth, when icebergs scrape against the ocean floor, they can rain boulders
down in clumps, which could explain boulder clusters up to about a mile (1.6
km) wide that scientists have seen on Mars. In addition, when icebergs roll
along the sea floor on Earth, they can generate strings of dents, perhaps
explaining the chains of craters seen on the Martian lowlands.

Seas
or oceans

If
there were icebergs, then there were open and sizable bodies of stable liquid water
on the surface of Mars, Fairen said.

"The
size of the water bodies may have ranged from several local seas to a single
hemispheric ocean, and they may have been continuous in time or episodic,"
he told SPACE.com.

Some
might suggest that the scattered boulders were deposited by so-called periglacial
processes, Fairen said ? that is, processes that take place at the edges of
glaciers. However, such processes cannot give a satisfactory explanation for
the boulder clusters that HiRISE also saw, he noted.

Others
have also suggested that the crater chains were formed by volcanic processes.

"But
our analyses can discard this hypothesis, especially because all the craters
within one chain are almost identical in shape and dimensions, and that's
neither expected nor usual in a volcanic process, but is expected if all the
craters in the chain are carved by the same iceberg," Fairen explained.

Fairen
added that scour marks some 0.6 to 3 miles (1 to 5 km) long seen in the
northern plains and Hellas Basin of Mars could be evidence of icebergs as well.
These could have been carved by the keels of icebergs scraping against the
ocean floor.

"The
scours are the most clear evidence for icebergs that we are finding," he
said.

Fairen
and his colleagues detailed their findings at the 2010 Astrobiology Science
Conference in April.